Pressure dependence of optical transitions in ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum wells

The effects of hydrostatic pressure on optical transitions in ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum wells (MQW’s) have been studied. The optical transition associated with confined electron and hole states in the MQW’s was found to shift linearly to higher energy with pressure but exhibit a significantly weaker pressure dependence compared to bulklike thick epitaxial-layer samples. Similar pressure coefficients obtained by both photomodulation and photoluminescence measurements rule out the possibility of the transition involving localized states deep in the band gap. We found that the difference in the compressibility of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ and GaN induces a tensile strain in the compressively strained ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ well layers, partially compensating the externally applied hydrostatic pressure. This mechanical effect is primarily responsible for the smaller pressure dependence of the optical transitions in the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ MQW’s. In addition, the pressure-dependent measurements allow us to identify a spectral feature observed at an energy below the GaN band gap. We conclude that this feature is due to transitions from ionized Mg acceptor states to the conduction band in the p-type GaN cladding layer rather than a confined transition in the MQW’s.